Method for producing concentrated nitric acid

ABSTRACT

Subconcentrated nitric acid of 80-90 percent by weight is effectively concentrated to concentrated nitric acid of more than 97 percent by weight using an integrated tower where both stripping of nitrogen peroxide and distillation of subconcentrated nitric acid are carried out. Thus, concentrated nitric acid of more than 97 percent by weight which contains nitrogen peroxide is fed into the upper section of a tower and nitric acid of 80-90 percent by weight is fed into the lower section of the tower and distillation is effected, thereby obtaining concentrated nitric acid of more than 97 percent by weight from the middle stage of the tower.

United States Patent Ohrui et al.

[ 1 Mar. 7, 1972 [54] METHOD FOR PRODUCING CONCENTRATED NITRIC ACID [72]Inventors: Tetsuya Ohrui, Niihama-shi; Yasuhito Sakakibara, Saijo-shi,both of Japan [21] Appl. No.: 825,319

Kramer et al.

Shaw

Randlett FOREIGN PATENTS OR APPLICATIONS 362,908 12/1931 Great Britain..23/ l 60 Primary Examiner-Oscar R. Vrtiz Assistant Examiner-G. O.Peters Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACTSubconcentrated nitric acid of 80-90 percent by weight is effectivelyconcentrated to concentrated nitric acid of more than 97 percent byweight using an integrated tower where both stripping of nitrogenperoxide and distillation of subconcentrated nitric acid are carriedout. Thus, concentrated nitric acid of more than 97 percent by weightwhich contains nitrogen peroxide is fed into the upper section of atower and nitric acid of 80-90 percent by weight is fed into the lowersection of the tower and distillation is effected, thereby obtainingconcentrated nitric acid of more than 97 percent by weight from themiddle stage of the tower.

4 Claims, 1 Drawing Figure PATENTEBMAR 7 I97 INVENTORS rsmayp mlmuATTORNEYS METHOD FOR PRODUCING CONCENTRATED NITRIC ACID This inventionrelates to a method for economically producing concentrated nitric acid,peroxide in that concentration of nitric acid and separation of nitrogenperoxide are carried out in a single integrated tower, and an apparatustherefor.

Methods for producing concentrated nitric acid which comprise separatingand liquefying nitrogen peroxide obtained by oxidizing ammonia with airand then reacting the said nitrogen peroxide with dilute nitric acid andoxygen under a high pressure have been known, for instance, as HOKOprocess, EMB- SEN process and so on.

According to such methods, a gaseous mixture obtained by oxidation ofammonia with air is washed with cold concentrated nitric acid to absorbnitrogen peroxide gas and then said concentrated nitric acid is heatedto strip and separate nitrogen peroxide dissolved therein. The separatednitrogen peroxide gas is cooled and liquefied and thereafter, togetherwith oxygen, is reacted with dilute nitric acid which is produced, forexample, during the condensation step of a gas obtained by oxidation ofammonia. Thus, concentrated nitric acid of more than 98percent by weightis produced directly in accordance with the reaction of 2N O +2H O+O4HNO However, since the reaction rate is extremely slow in particular atthe last period of the reaction and a long time is required forcompletion of the reaction, it has been necessary to use commercialoxygen as an oxygen source and to raise the reaction pressure to morethan 50 atmospheres.

However, the use of expensive oxygen and increased reaction pressurecause an increase in cost of raw materials and power, thereby spoilingprocess economies. In order to overcome the above defects, a method hasbeen proposed wherein the concentration of produced nitric acid is keptat 80-90 percent by weight and then this nitric acid is concentrated to98 percent by weight and then this nitric acid is concentrated to 98percent by weight.

According to said method, since the severity of the required reactionconditions is extremely mitigated, air can be used as the oxygen sourceand the reaction pressure can be reduced to less than atmospheres. Thus,said method brings on great economic benefits as to these points.However, nitric acid of 80-90 percent by weight produced must bedistilled to more than 97 percent by weight. Consequently, a heat mediumsuch as steam is required as a heat source for said distillation. Thus,said economic benefits are nearly offset.

lt is therefore the object of this invention to provide an effectivemethod of producing concentrated nitric acid.

We studied the operating conditions and the heat balance of strippingtower for nitrogen peroxide (referred to as a stripping towerhereinafter) and the distilling for produced nitric acid (referred to asa distilling tower hereinafter) in the process as mentioned above. Asthe result of this study, we found that under certain conditions theheating calories in the reboiler of the stripping tower and coolingcalories in the condenser of the distilling tower are nearly the same.We further found that the temperatures and compositions of contents inboth the reboiler and condenser are the same and so a combination of thetwo towers into one by integrating them is possible.

This invention relates to a method for producing concentrated nitricacid which comprises feeding concentrated nitric acid of more than 97percent by weight which contains nitrogen peroxide into the uppersection of a tower and simultaneously feeding nitric acid of 80-90percent by weight into the lower section of the tower to effectstripping and distillation and removing nitrogen peroxide from the topof the tower, concentrated nitric acid of more than 97 percent by weightfrom the middle part of the tower and nitric acid of 68-72 percent byweight from the bottom part.

In the strippingand distilling-tower of this invention, steam is fed asa heat medium to the reboiler provided at the bottom of the tower. Sincethe calorie required for distillation is nearly the same as that forstripping as mentioned above, the calorie to be supplied is merely thatrequired for stripping of nitrogen peroxide. That is, the calorie usedfor stripping is also used for distillation and no additional calorie isrequired.

The especially important point in this invention is the balance ofcalories between the upper section and the lower section of the tower.That is, it is important that the calories required in the upper sectionand the lower section of the tower are nearly the same. For thiscondition to exist, the concentration of nitrogen peroxide in theconcentrated nitric acid its feed rate to the upper section of the towerand the concentration and feed rate of the produced nitric acid{subconcentrated nitric acid (-90 percent by weight)} to the lowersection of the tower should be controlled.

As an example, the production of l ton,of nitric acid of 98 percent byweight/hr. is illustrated below. In order to distill 1.87 ton/hr. ofnitric acid of percent by weight with seven theoretical stages and at areflux ratio of 0.4 it is necessary to condense 1.4 ton/hr. of vapor ofnitric acid of 98 percent by weight, said condensation generates a heatof condensation of 2.2Xl0 kilocalories/hr. From 3.3 tons/hr. of nitricacid of 98 percent by weight containing 25 percent by weight of nitrogenperoxide, the nitrogen peroxide can be stripped at a reflux ratio of 0.1with four theoretical stages using said calories developed during saidcondensation. Furthermore, the amount of nitrogen peroxide to bestripped corresponds to the amount required to produce 1 ton of nitricacid of 98 percent by weight. Therefore, when the concentration ofnitric acid produced in the nitric acid producing step is more thanabout 85 percent by weight, the calories required for stripping and thatfor distillation are nearly the same so that the produced nitric acidcan be concentrated into more than 98 percent by weight by utilizingsaid calories for a dual purpose. (The term ton as used herein refers tothe metric system.)

It is obvious that since nitric acid of 85 percent by weight can beeasily produced using air as an oxygen source under a reaction pressureof less than l0 atmospheres, the cost of production is extremelyimproved.

However, if the concentration of the produced nitric acid is less thanabout 85 percent by weight, the calories required for distillationincreases due to the increase in reflux ratio and in feed rate andtherefore exceed the calories required for stripping. Thus, the benefitsof this invention are decreased by the difference between the bothcalories.

Further, when the concentration of the produced nitric acid is more thanabout 85 percent by weight, there is no special disadvantage with regardto calorie, but since the conditions for producing nitric acid becomemore severe, disadvantage arises therefrom. Therefore, when thestripping and distillation conditions are most economically provided,the most preferably concentration of the produced nitric acid is 83-87percent by weight, especially about 85 percent by weight.

This invention will be explained with reference to the attached drawingbelow.

The gas produced by oxidation of ammonia is fed into absorption tower 2through line 1 and is washed with concentrated nitric acid of 98 percentby weight fed through line 3. The waste gas from a nitric acid producingtower is also introduced into absorption tower 2 through line 4 andunreacted nitrogen peroxide is recovered.

The absorption tower 2 is an aluminum made plate tower and is operatedat a room temperature under a pressure 3-7 atm. The concentrated nitricacid discharged from the tower bottom contains 20-30 percent by weightof nitrogen peroxide and is introduced into upper section 7 of thestrippingand distilling-tower through line 6.

The strippingand distilling-tower is an aluminum and titanium made platetower and is operated at an atmospheric pressure or 2-3 atm.

Said concentrated nitric acid is directly heated with vapor of nitricacid raising from lower section 8 of the strippingand distilling-towerthrough chimney 25 and releases nitrogen peroxide. On the other hand,said vapor of nitric acid from which sensible heat is removed bystripping condenses and is refluxed to the top of lower section 8 fromthe bottom of the upper section of the strippingand distilling-towerthrough lines 18 and 19. The preferable reflux ratio is 0.2-0.5.Released nitrogen peroxide contains small amount of nitric acid and thisis removed therefrom by partial condenser 9. At this time, a part ofnitrogen peroxide is also condensed. That is, in condenser 9, a part ofnitrogen peroxide and nitric acid coexisting therewith are condensed.Thereafter, said nitrogen peroxide is fed to nitric acid producing tower14 through line 11 and compressor 12 where the pressure is raised to 4-8atm. Nitric acid producing tower 14 is a titanium made packed tower andnitricacid of 68-72 percent by weight, nitrogen peroxide gas and air areintroduced into the top, the middle stage and the bottom through lines15, 13 and 16, respectively. In the upper stage, nitric acid is producedand in the lower stage, nitrogen peroxide dissolved in nitric acidproduced is stripped. Nitric acid of about 83-87 percent by weightproduced at reaction condition of pressure of 48 atm. and temperature of5080 C. was discharged from the bottom and is fed to lower section 8 ofthe strippingand distillingtower through line 17. This nitric acid isheated at reboiler at the bottom and simultaneously cooled withconcentrated nitric acid refluxed through lines 18 and 19 from the uppersection, and is separated into nitric acid of 98 percent by weight andnitric acid of 68-72 percent by weight by so-called distillation action.The nitric acid of 98 percent by weight is taken out as a product fromthe bottom of the upper section 7 of the strippingand distilling-towerthrough line 18, pump 20, cooler 21 and line 22. A part of this nitricacid is recycled to absorption tower 2 through line 3. On the otherhand, the nitric acid of 68-72 percent by weight is recycled toproducing tower 14 through line 23, pump 24 and line and is reused therefor producing nitric acid.

As mentioned above, according to this invention, nitric acid of about 85percent by weight which is easily produced is concentrated into nitricacid of more than 97 percent by weight by distillation andsimultaneously therewith, nitrogen peroxide which is necessary forproducing nitric acid is separated from nitric acid containing nitrogenperoxide. Thus, the method of this invention is a very effective methodthermally.

EXAMPLE 800 kg./hr. of nitric acid of 85 by weight (50C.) was introducedinto the ninth stage (from the bottom) of a strippingdistilling tower (abubble cap tower) of 30 cm. in diameter having 22 stages (8stages inupper section and l4 stages in lower section) and 1,300 kg./hr. (30C.)of concentrated nitric acid of 98 percent by weight having absorbed 25percent by weight of nitrogen peroxide was introduced to the third stagefrom the top.

About 250 kg./hr. of steam (4 atm. saturated) was supplied into thereboiler at the bottom of the tower as a heat source. In this case,reflux of the concentrated nitric acid was 170 kg./hr. and that ofnitrogen peroxide by partial condensing was 3.4 kg./hr. The pressure atthe tower top was atmospheric pressure, the temperature at the tower topwas 50 C. and the temperature at the bottom was 140 C.

From the top, 340 kg./hr. of nitrogen peroxide containing nitric acid of5 percent by weight was taken out and from the bottom, nitric acid of 70percent by weight was taken out at 365 kg./hr. From the middle stage,nitric acid of 98 percent by weight was taken out at a rate of 1,395kg./hr. When stripping and distilling were carried out in separatetowers, the flow rate of steam used was 400 kg./hr. under said flowrates. Thus, it is clear that according to this invention, the amount ofsteam used can be sharply reduced.

What we claim is:

1. A method for producing concentrated nitric acid which comprises a.feeding a nitric acidcontaining more than 97 percent b weight of HNO andfrom 20 to 30 percent by weight of nitrogen peroxide to an uptpersection of a zone, b. simu taneously therewith eeding nitric acid offrom to percent by weight to a lower section of said zone, said upperand lower sections being separated from each other, but being in opencommunication with each other through a narrow passageway,

c. distilling said nitric acid in said lower section to produce abottoms of nitric acid of from 68 to 72 percent by weight and togenerate vapors of nitric acid,

d. allowing said generated vapors to travel through said passageway intosaid upper section,

e. contacting 4vapors of said nitric acid generated in said lowersection with said N 0 -containing nitric acid in said upper section tostrip out nitrogen peroxide therein by heating said N o -containingnitric acid with said vapors of nitric acid,

removing said nitrogen peroxide from the top of said upper section, saidnitric acid of from 68 to 72 percent from the bottom of said lowersection and concentrated nitric acid of more than 97 percent by weightfree from N 0 from the bottom of said upper section,

refluxing a portion of said concentrated nitric acid withdrawn to saidlower section of a said zone,

. producing nitric acid of from 80 to 90 percent by weight from saidnitrogen peroxide removed from the top of said upper section of saidzone, from said nitric acid removed from said bottom of said lowersection of said zone and from air,

'. feeding the nitric acid produced in step (h) to said lower section ofsaid zone according to step (b),

j. feeding the nitrogen peroxide gas unreacted in said nitric acidproducing step (h) and a nitrogen peroxide gas produced by oxidation ofammonia to an absorption tower,

k. feeding a portion of a said concentrated nitric acid removed from thebottom of said upper section of said zone to said absorption tower,

. absorbing said nitrogen peroxide gases of step (j) into saidconcentrated nitric acid of step (k) in said absorption tower to producethe concentrated nitric acid solution containing more than 97 percent byweight of HNO; and from 20 to 30 percent by weight of nitrogen peroxide,and

in. removing said concentrated nitric acid solution produced in step (1)from said absorption tower and feeding same to said upper section ofsaid zone in step (a).

2. A method according a claim 1 wherein the concentration of said nitricacid of steps (b) and (h) is from 83 to 87 percent by weight.

3. A method according to claim 2 wherein said concentration is about'85percent.

4. A method according to claim 1 wherein the reflux ratio in step (g) isfrom 0.2 to 0.5.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,647,376 Dated March 7, 1972' l v t r( Tetsuya Ohrui et a1.

It is certified that error appears in the above-identified patentandthat said Letters Patent are hereby corrected as shown below:

Please insert the following claim for priority! -Japanese No. 37444/68of May 31, 1968-- Signed and sealed this 15th day of August 1972.

(SEAL) Attest:

EDWARD I'LFLETCHERJR. ROBERT GOTTSCHALK Attesting Officer I Commissionerof Patents FORM PO-105O (10-69) USCOMWDC 603mm .5. GOVERNMENT PRIN INGOFF Ci I969 0-356-33

2. A method according a claim 1 wherein the concentration of said nitricacid of steps (b) and (h) is from 83 to 87 percent by weight.
 3. Amethod according to claim 2 wherein said concentration is about 85percent.
 4. A method according to claim 1 wherein the reflux ratio instep (g) is from 0.2 to 0.5.